During the past ten years I have carried out extensive studies of the Mg2 ion-ATPase and F-actin-activated ATPase activity of skeletal muscle myosin. I have now prepared highly purified heart myosin to study the kinetic properties of its Mg2 ion-ATPase and F-actin-activated ATPase activities. The maximum binding values and association constants of heart myosin for ATP and for adenylyl imidodiphosphate, an analog of ATP not hydrolyzed by myosin, are being determined. I shall study the 0inetics of the superprecipitation activity of reconstituted actomyosin prepared from pure heart myosin and pure F-actin. Finally, the ability of heart myosin to react with radioactive ATP and radioactive adenylyl imidodiphosphate to form an (NH4)2SO4-precipitable complex like the skeletal muscle myosin does will be determined. Identical studies will also be carried out on highly purified blood platelet myosin. These studies will provide a better understanding of the functions of myosin in heart and in platelets.